1. Field of the Invention
The present invention relates to an air conditioner, and more particularly to a dual-purpose cooling/heating air conditioner and to a control method thereof for maintaining a degree of refrigerant superheat at an appropriate value to thereby increase the heating efficiency and to prevent the refrigerant compressor from being damaged as well.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a typical conventional dual-purpose cooling/heating air conditioner for illustrating a refrigerant cycle thereof.
In FIG. 1, during a heating cycle the high-temperature and high-pressure refrigerant compressed by a compressor 1 is infused into an indoor heat exchanger 4 through a four-way valve 10.
The high-temperature and high-pressure refrigerant infused into the indoor heat exchanger 4 emits heat into the indoors by way of an indoor fan 12 to thereby become condensed.
The refrigerant condensed by the indoor heat exchanger 4 becomes saturated under low pressure by means of passing through a pressure reducer 3 and is discharged from the pressure reducer.
The refrigerant discharged by the pressure reducer 3 is infused into an outdoor heat exchanger 2 and absorbs ambient heat from the outdoors by way of an outdoor fan 11 to thereby become evaporated.
The refrigerant evaporated by the outdoor heat exchanger 2 is infused into an accumulator 5 through the four-way valve 10.
The accumulator 5 prevents liquid refrigerant from being infused into the compressor 1, to thereby infuse only the evaporated refrigerant into the compressor 1.
Meanwhile, a reverse cycle of the aforesaid sequence is performed during a cooling and de-frosting operation.
The four-way valve 10 sends the refrigerant coming from the compressor 1 to the indoor heat exchanger 4 during the heating operation, and sends the same to the outdoor heat exchanger 3 during the cooling or de-frosting operation.
Furthermore, reference numeral 13 designates a non-return valve for passing the refrigerant during the cooling operation and for not passing the refrigerant during the heating operation.
FIG. 2 is a diagram plotting compressor speed in revolutions per minute (rpm) against the open air temperature in a conventional dual-purpose cooling/heating air conditioner.
When the open air temperature is below Ta(approximately 3 degrees below zero celsius, which is the open air temperature where the heating capacity and heating load coincide when the compressor is operated at maximum speed) during the heating, the compressor 1 is operated at the maximum speed while, when the open air temperature is above Tr(approximately) 25 degrees celsius, which is the open air temperature where the operation of the compressor is unnecessary), the operation of the compressor is stopped.
FIG. 3 is a diagram plotting heating capacity and heating load against the open air temperature in a conventional dual-purpose cooling/heating air conditioner.
In FIG. 3, it should be noted that the lower the open air temperature, the heavier the heating load, and the higher the open air temperature, the more increased is the heating capacity in relation to the revolution speed of the compressor.
In other words, when the open air temperature is above Tr(approximately 21 degrees celsius), the indoor temperature can be increased to a temperature a user wants even though the revolution speed of the compressor is minimized.
If the open air temperature is above To(approximately 7 degrees celsius), the indoor temperature can be increased to a temperature the user wants with the revolution speed of the compressor at a rated speed.
If the open air temperature is above Ta, the revolution speed of the compressor is increased to the maximum to thereby make the indoor temperature reach a temperature the user wants. However, if the open air temperature is below Ta, the indoor temperature can not be increased to a temperature the user wants even though the revolution speed of the compressor is maximized, where, the open air temperatures Ta, To, Tr have the relations of Ta&lt;To&lt;Tr.
As described above, in the conventional dual-purpose cooling/heating air conditioner there has been a problem in that the indoors can not be heated up to a temperature the user wants even though the compressor is operated at the maximum speed due to lack of a sufficient heat source at low outdoor temperature.
FIG. 4 is a pressure(p)-enthalpy(h) curve diagram of a conventional dual-purpose cooling/heating air conditioner.
In other words, if a refrigerant having T2 as a suction temperature at the compressor inlet is compressed, the enthalpy is increased to thereby make the refrigerant discharge temperature at the compressor outlet reach T1.
The refrigerant discharged from the compressor 1 which is in an evaporated state at T1 enters the indoor heat exchanger 4 (or condenser) to thereafter emit heat into the indoors, and the refrigerant thereby becomes condensed into a liquid state and discharged. The refrigerant becomes low in pressure at the pressure reducer 3 (or expansion apparatus) to thereafter be discharged in a mixed state of liquid and gas.
The refrigerant discharged from the pressure reducer 3 absorbs heat from the outdoor heat exchanger 2 and becomes gaseous when the absorption temperature of the refrigerant at the compressor inlet reaches T2.
However, if the refrigerant does not absorb enough heat from the outdoor heat exchanger 2 so that the temperature of the refrigerant remains below a saturated temperature Ts, the refrigerant to be infused into the compressor 1 is in the mixed state of gas and liquid.
At this moment, if the liquidized refrigerant is infused into the compressor 1, there arises a phenomenon where incompressible liquid changes into gas instantly when compressed, so that the refrigerant gets increased in volume to thereby cause damage to vanes and rollers comprising the compressor.
Accordingly, it is a role of the accumulator 5 to prevent the liquid refrigerant from being infused into the compressor 1 and to infuse only the evaporated refrigerant into the compressor 1.
At this location, a section from Ts to T2 is called a degree of superheat (SHs; see FIG. 4) and the ideal degree of superheat (SHs=T2-Ts) in the conventional dual-purpose cooling/heating air conditioner is approximately 6 degrees celsius.
However, because the conventional dual-purpose cooling/heating air conditioner has lacked in the degree of superheat due to want of a heat source at a low outdoor temperature, the refrigerant can not be evaporated fully within the outdoor heat exchanger 2, so that the refrigerant in the mixed state of liquid and gas has been infused into the accumulator 5.
If the mixed refrigerant of liquid and gas is infused into the accumulator 5, the accumulator 5 discharges only the gaseous refrigerant to the compressor 1 and the liquid refrigerant remains to thereby be accumulated.
If the liquidized refrigerant is accumulated in the accumulator 5, there occurs a phenomenon where the liquidized refrigerant and its lubricating oil are separated at a border to thereby cause the compressor 1 to operate improperly.
In other words, in order to operate the compressor 1 smoothly, oil is injected into the compressor 1 and part of the oil is discharged with the refrigerant.
If the liquidized refrigerant is accumulated in the accumulator 5, the oil is not retrieved into the compressor 1.
As seen from the foregoing, the conventional dual-purpose heating/heating air conditioner has a problem in that it has deteriorated cooling capacity due to lack of a heat source at low outdoor temperature (open air temperature) and the refrigerant is not fully evaporated to thereby cause the compressor to become damaged.